Neurotrophins, a family of highly conserved growth factors, are best characterized for critical roles in sculpting the developing peripheral and central nervous system through contrasting actions that regulate neuronal differentiation, survival and cell death. More recent studies reveal important roles in the adult in modulating behavior, synaptic plasticity, and in injury protection. Neurotrophins mediate these effects through two receptors; the Trk receptor tyrosine kinases that mediate survival, plasticity and behavioral effects, and p75, a TNF receptor family member that can induce survival and apoptotic actions. Neurotrophins are initially synthesized as precursors (proneurotrophins, proNTs) that are cleaved to mature forms, long thought to be the biologically active moiety. However proNTs can be released by cells, and our studies indicate that they selectively bind p75 but not Trk receptors to promote neuronal apoptosis. We have uncovered a third receptor, sortilin, which binds to the pro-domains of neurotrophins, and forms a co-receptor complex with p75 to convey apoptosis. In vivo models suggest that proNGF is an inducible cytokine which is upregulated following injury, and strategies that impair proNGF function in the injured CNS are neuroprotective. In contrast, proBDNF is an endogenous ligand that is released from central neurons, and must be locally processed at the synapse to generate mature BDNF to induce L-LTP. The long-term goal of our work is to understand the biochemical and molecular basis of neurotrophin function. We propose three interrelated approaches to identify the mechanisms by which proNTs mediate distinct actions. Specifically we propose to: (a) define the signaling pathways that are unique to proNTs to initiate p75-mediated cell death. Using cells which co-express p75, TNF receptor and Fas, we will identify the "death adaptor complex" that is selectively recruited to ligand-activated p75 to determine the specificity of apoptotic signaling, (b) Analyze the biological consequences of ectodomain shedding of the sortilin receptor, which is induced upon proNT binding. We will identify the proteases that mediate shedding, and examine if sortilin ectodomain acts as a decoy receptor to abrogate apoptosis, or whether shedding is required to initiate cell death, (c) Genetically dissect the actions of proNGF and proBDNF in the central and peripheral nervous system using inducible, gene replacement strategies. These will be used to evaluate the consequences of proNGF and proBDNF expression in (i) sculpting peripheral and central nervous systems by mediating developmentally regulated cell death, (ii) altering synaptic plasticity in the juvenile and adult hippocampus, and (iii) mediating apoptosis in the adult nervous system.